Recent advancements in the electronics industry are remarkable and functional devices depend on new principles are being developed one after another. This requires upgrading of the purity of the materials of which these devices are made. High-purity tantalum and tantalum pentoxide are materials of increasing interest for fabricating surface acoustic wave filters, pyroelectric infrared sensors, optoelectronic devices and VLSI devices, and these materials are required to have a very high purity.
For example, surface acoustic wave filters and pyroelectric infrared sensors are made of lithium tantalate single crystal, and it is required to use high-purity tantalum pentoxide which does not contain iron, silicon, aluminum and niobium that will hinder higher yield and performance. For production of optical glass and optoelectronic devices such as optical switches and optical waveguides, it is also indispensable that the high-purity tantalum pentoxide used does not contain silicon or transition elements (e.g., iron, chromium and nickel) that would cause deteriorated performance, particularly increased light absorption loss.
Tantalum pentoxide is also used as a material for capacitors in a VLSI device and is required to be free from such alkali metal impurities as sodium and potassium that would deteriorate the performance of the device. This requirement is barely met by the conventional purity of two to three nines (99 to 99.9%) and considerable effort is being made to attain purities of the order of four to six nines (99.99 to 99.9999%).
Three methods, i.e., distillation, extraction and use of an anion exchange resin, are generally employed to obtain pure tantalum from crude tantalum or tantalum compounds containing impurities such as silicon, iron, copper, aluminum, niobium and calcium.
The distillation technique is described in, for example, German Pat. Nos. 1,017,601 and 1,056,105; this method depends on the difference between the boiling points of chlorides formed by chlorinating a mixture of tantalum or a tantalum compound and, for example, carbon with a chlorine gas supplied at about 600.degree. to 800.degree. C. However, the difference between the boiling point of tantalum chloride (239.degree. C.) and that of niobium chloride (249.degree. C.) is so small for industrial purposes that considerable difficulty is encountered in separating these chlorides by fractional distillation. Furthermore, if a chloride having a boiling point even closer to that of tantalum chloride is present, it is substantially impossible to separate the two chlorides and, as a result, high-purity tantalum pentoxide cannot be obtained.
The second method, extraction, is carried out by the following procedures: tantalum or a tantalum compound is dissolved in hydrofluoric acid; the resulting solution is brought into contact with an organic solvent made of an aliphatic ketone such as methyl ethyl ketone, cyclohexanone or methyl isobutyl ketone, whereby the tantalum in the acidic aqueous solution is extracted into the organic solvent, while the niobium extracted simultaneously into the organic solvent is removed by back extraction with dilute sulfuric acid or a sulfuric acid solution containing hydrofluoric acid; subsequently, the organic solution mainly containing tantalum is brought into contact with water or an aqueous solution containing one or more of dilute sulfuric acid, ammonium sulfate and ammonium fluoride, whereby the tantalum is extracted from the organic solvent into the aqueous solution, which is then hydrolyzed with aqueous ammonia to form a precipitate of tantalum hydroxide [Ta(OH).sub.5 ]; and the precipitate is recovered by filtration and calcined in air to form tantalum pentoxide [Ta.sub.2 O.sub.5 ].
Alternatively, the aqueous solution of tantalum in hydrofluoric acid is brought into contact with an organic solvent made of an aliphatic phosphoric acid compound such as tributyl phosphate or bis(2-ethylhexyl)phosphoric acid, whereby the tantalum is extracted from the acidic aqueous solution into the organic solvent, which is then brought into contact with nitric acid so as to remove niobium by back extraction (see, for example, Japanese Patent Publication Nos. 13607/1963 and 18061/1963).
According to the method proposed in Japanese Patent Publication No. 29256/1983, the aqueous solution of tantalum in hydrofluoric acid is brought into contact with an organic solvent made of hydrocarbons of low polarity containing a tertiary alkylamine with a high molecular weight, whereby the tantalum is extracted from the acidic aqueous solution into the organic solvent, and subsequently niobium is removed from the organic solvent by back extraction with the tantalum-containing aqueous solution of hydrofluoric acid.
The principle of tantalum purification employed in these extraction techniques is the small difference that exists between tantalum and an impurity such as niobium with respect to affinity with the organic solvent, so the niobium cannot be completely removed by back extraction with dilute sulfuric acid or the aqueous solution of hydrofluoric acid and the tantalum pentoxide finally obtained contains several hundred ppm of niobium and is unable to provide a purity higher than the order of two to three nines (99 to 99.9%). For the small difference between tantalum and an impurity with respect to affinity with the organic solvent, if in order to obtain high-purity tantalum, the number of back extraction cycles is increased, the amount of tantalum transferred from the organic solvent into the back extract is also increased leading to causing a substantial decrease in the percent recovery of tantalum.
The third approach for tantalum purification depends on the use of an anion exchange resin: crude tantalum or an impure tantalum compound is dissolved in, for example, hydrofluoric acid; the resulting acidic aqueous solution containing tantalum is passed through a column packed with an anion exchange resin of the strongly basic quaternary ammonium type, so as to adsorb tantalum selectively on the anion exchange resin; thereafter, a metallic impurity that has been simultaneously adsorbed on the resin is removed by flushing with a strongly acidic aqueous solution containing hydrofluoric acid; and subsequently, tantalum is recovered by eluting the column with a mixed aqueous solution composed of hydrofluoric acid, aqueous ammonia and an ammonium salt. This method is described in, for example, Japanese Patent Application (OPI) No. 114831/1981, which states that the method is capable of providing high-purity tantalum pentoxide containing no more than a few ppm of tantalum of such metallic impurities as iron, silicon, copper, calcium, niobium, and aluminum. (The term "OPI" as used herein refers to a "published unexamined Japanese patent application".)
However, this method is not highly suitable for practical applications because the amount of tantalum that can be processed by a single cycle of column passage, flushing and elution is determined by the fairly small exchange capacity of the anion exchange resin (e.g., 1.0 to 1.5 mol Ta/liter-resin), and in order to apply this method on a commercial scale, a large quantity of the resin must be employed while the aforementioned complex batch operations are repeated quite a number of times.
Further problems arise from hydrolysis of tantalum during elution to cause precipitation of tantalum hydroxide on the anion exchange resin. First of all, in order to avoid formation of such precipitate, tantalum must be recovered by slowly dissolving it in an eluant but, then, this necessitates the use of an increased amount of eluant and prolongs the period of elution. Secondly, a large quantity of fluorine-containing effluents that require certain waste treatments are formed when tantalum is subsequently treated with aqueous ammonia to be precipitated in the form of tantalum hydroxide for recovery by filtration.